Still, the administration appeared caught off guard by the speed of the reaction as word leaked out late Wednesday.

Undersecretary of State Ellen O. Tauscher and two high-level Pentagon officials made a whirlwind trip Wednesday night and Thursday to Poland, the Czech Republic and NATO to brief allies. Secretary of State Hillary Rodham Clinton called several counterparts and Obama did the same, a State Department official said Thursday.

The move is likely to please some in Eastern Europe, who had been angered by Bush's plans for their countries. When Obama visited Prague in April, several hundred Czechs marched in the capital to decry the missile shield proposal, carrying balloons and placards, including one that read: "Yes We Can -- Say No to Missile Shield."

But at home, Obama's decision sparked immediate condemnation from Republicans in Congress, who accused the administration of abandoning America's allies and putting the country's security at risk. House Minority Leader John A. Boehner (Ohio) said in a statement that the move "does little more than empower Russia and Iran at the expense of our allies in Europe. It shows a willful determination to continue ignoring the threat posed by some of the most dangerous regimes in the world."

That concern was echoed by Obama's chief rival during the 2008 campaign, Sen. John McCain (Ariz.), who called the move away from a missile system designed to counter long-range weapons "seriously misguided."

"Given the serious and growing threats posed by Iran's missile and nuclear programs, now is the time when we should look to strengthen our defenses, and those of our allies," McCain said in a statement. "Missile defense in Europe has been a key component of this approach."

In his briefing, Gates anticipated those criticisms and fired back strongly. "Those who say we are scrapping missile defense in Europe are either misinformed or misrepresenting the reality of what we are doing," Gates said. "The security of Europe has been a vital national interest of the United States for my entire career. The circumstances, borders and threats may have changed, but that commitment continues."

For Gates, the president's decision is an especially dramatic reversal. In December 2006, shortly after assuming office as Bush's defense secretary, Gates recommended the missile defense system based in the Czech Republic and Poland to protect against the threat of longer-range missiles that Iran was developing.

White House officials said Gates's support of the new approach gives it credibility and serves to undermine the accusations made by the president's Republican adversaries.

Critics of the Bush defense plan have long said it addressed a threat that did not exist, using missiles that might not work. That system was aimed at shooting down long-range ballistic missiles, which Iran is not expected to have until at least 2015, according to arms-control experts. The system was not intended to deal with Iran's medium-range missiles, which are capable of hitting Turkey and the edge of Europe.

In addition, the two-stage missiles that were supposed to be based in Poland as part of the Bush-era shield plan have not been tested.

As described by Gates and his top generals, Obama's new missile defense plan will unfold in three stages. By 2011, the Pentagon will deploy Navy Aegis ships equipped with SM-3 interceptors in the eastern Mediterranean.

A second phase in about 2015 will field an upgraded, land-based SM-3 in allied countries, and discussions are underway with Poland and the Czech Republic on basing the missiles in their territory, Gates said. In 2018, the third phase will deploy a larger and more capable missile, which will allow the defense shield to protect Europe and the United States against short- and intermediate-range rockets and, eventually, intercontinental ballistic missiles.

Tyson reported from the Pentagon. Staff writers Mary Beth Sheridan and Walter Pincus in Washington, correspondent Philip P. Pan in Helsinki and special correspondent Shannon Smiley in Berlin contributed to this report.